PII S0360-3016(00)00748-3 PHYSICS CONTRIBUTION PHYSICAL ASPECTS OF A REAL-TIME TUMOR-TRACKING SYSTEM FOR GATED RADIOTHERAPY HIROKI SHIRATO, M.D.,* SHINICHI SHIMIZU, M.D.,* TATSUYA KUNIEDA, ² KEI KITAMURA, M.D.,* MARCEL VAN HERK,PH.D., ‡ KENJI KAGEI, M.D.,* TAKESHI NISHIOKA, M.D., SEIKO HASHIMOTO, M.D.,* KATSUHISA FUJITA, R.T.,* HIDEFUMI AOYAMA, M.D.,* KAZUHIKO TSUCHIYA, M.D., KOHSUKE KUDO, M.D.,* AND KAZUO MIYASAKA, M.D.* *Department of Radiation Medicine, Hokkaido University School of Medicine, Sapporo, Japan; ² Mitsubishi Electronics Company, Ltd., Tokyo, Japan, ‡ Department of Radiotherapy, The Netherlands Cancer Institute/Antoni van Leeuwenhoek Huis, Amsterdam, The Netherlands Purpose: To reduce uncertainty due to setup error and organ motion during radiotherapy of tumors in or near the lung, by means of real-time tumor tracking and gating of a linear accelerator. Methods and Materials: The real-time tumor-tracking system consists of four sets of diagnostic X-ray television systems (two of which offer an unobstructed view of the patient at any time), an image processor unit, a gating control unit, and an image display unit. The system recognizes the position of a 2.0-mm gold marker in the human body 30 times per second using two X-ray television systems. The marker is inserted in or near the tumor using image guided implantation. The linear accelerator is gated to irradiate the tumor only when the marker is within a given tolerance from its planned coordinates relative to the isocenter. The accuracy of the system and the additional dose due to the diagnostic X-ray were examined in a phantom, and the geometric performance of the system was evaluated in 4 patients. Results: The phantom experiment demonstrated that the geometric accuracy of the tumor-tracking system is better than 1.5 mm for moving targets up to a speed of 40 mm/s. The dose due to the diagnostic X-ray monitoring ranged from 0.01% to 1% of the target dose for a 2.0-Gy irradiation of a chest phantom. In 4 patients with lung cancer, the range of the coordinates of the tumor marker during irradiation was 2.5–5.3 mm, which would have been 9.6 –38.4 mm without tracking. Conclusion: We successfully implemented and applied a tumor-tracking and gating system. The system signif- icantly improves the accuracy of irradiation of targets in motion at the expense of an acceptable amount of diagnostic X-ray exposure. © 2000 Elsevier Science Inc. Radiotherapy, Setup error, Organ motion, Real-time tracking, Gating, Pattern recognition. INTRODUCTION Organ motion and setup error are important uncertainties in external radiotherapy for extracranial diseases. The plan- ning target volume (PTV) must have a larger margin for moving tumors compared to static tumors with the same clinical target volume (CTV). One approach to improve the treatment accuracy for targets in or near the lung, is to gate the accelerator to the respiratory motion (1–5). We have previously reported that lung tumors follow complicated three-dimensional (3D) tracks (6, 7). By detecting the internal movement of a tumor in real time during treatment and by using this information to gate the accelerator, the accuracy of irradiation can be improved (8). We have developed a system which is capable of tracking the 3D position of a metallic marker in the body in real time by means of two sets of diagnostic X-ray imaging equip- ment. This system is in clinical use for patients with ex- tracranial diseases (9). The linear accelerator is gated by the real-time tumor-tracking system to irradiate the target vol- ume only when the internal marker is near its planned position. About 30 patients with lung tumors, liver tumors, and other diseases have been treated using this system. The aims of this study are to determine the accuracy of the system and the additional dose due to the diagnostic X-ray, and to determine its performance during clinical use. METHODS AND MATERIALS Hardware The real-time tumor-tracking system was constructed by Mitsubishi Electronics Co., Ltd., Tokyo, for this study. It Reprint requests to: Dr. Hiroki Shirato, Department of Radiol- ogy, Hokkaido University School of Medicine, North-15 West-7, Kita-ku, Sapporo, Japan 060-8638. E-mail: hshirato@radi.med. hokudai.ac.jp This work was supported by a grant from the Ministriy of Educational, Science, Sports, and Culture, Japan (No. 12470182) and the Japanese Association for Nuclear Technology in Medicine. Accepted for publication: 14 June 2000. Int. J. Radiation Oncology Biol. Phys., Vol. 48, No. 4, pp. 1187–1195, 2000 Copyright © 2000 Elsevier Science Inc. Printed in the USA. All rights reserved 0360-3016/00/$–see front matter 1187